1. Field of the Invention
The present invention relates generally to belt driven magnetic tape cartridges and more particularly to belt driven magnetic tape cartridges especially adapted for high density digital data recording on thin magnetic tapes.
2. Description of the Prior Art
Belt driven magnetic tape cartridges are widely used in the computer industry for digital data recording. One particular form of such cartridge is taught by the United States Pat. No. 3,692,255, issued to Robert A. Von Behren. The Von Behren tape cartridge has two reel hubs onto which magnetic recording tape is wound. The hubs are driven by a flexible driving belt which extends around two guide rollers and a driving roller such that the belt contacts the magnetic tape pack formed about the reel hubs. Rotation of the driving roller drives the belt and thereby causes the tape to move between the two reel hubs. This places the tape traveling between the two reel hubs under tension. This cartridge construction permits low power bidirectional drive and rapid acceleration and deceleration of the tape by a single reversible driving means.
For several years, manufacturers of tape recording and reproducing mechanisms have been attempting to manufacture and sell mechanisms capable of recording and reproducing digital data on this style of belt driven magnetic tape cartridge at capacities in the order of 6,400 linear bits per inch. These attempts have not been commercially successful. A key reason for the lack of success is due to the fact that the currently available tape cartridges develop too much flutter after having been placed in service for short periods of time. In connection with belt driven tape cartridges, "flutter" means high frequency variations in the linear velocity at which the tape moves past the recording and/or reproducing means. One manufacturer of such a digital recording and reproducing mechanism requires that the flutter, i.e. variations in the linear velocity of the tape, be less than 0.8% of the average linear velocity. Though some currently commercially available belt driven tape cartridges will meet this specification when initially placed in service, they do not hold such tolerances after the cartridge is in use for a short time period. For example, after approximately forty repetitive movements of a short section of the tape in such a cartridge along the same section of that tape, the flutter will rise to approximately 5% or greater.
The cause of this flutter is inherent in the structure of these belt driven tape cartridges as taught by Von Behren. The belt drive is designed such that the segment of the tape traveling between the tape segments wound onto the reel hubs is continually under tension. The tension in the tape occurs because the belt driving roller and the two belt guiding rollers are especially adapted to tension the tape so as to cause the take-up reel hub to wind the tape at a faster rate than the rate at which the tape leaves the supply reel hub. Two effects created by this design are first, the tension in the tape increases as tape is moved from a full supply reel hub to an empty take-up reel hub, and second, the tape wound onto a reel hub is under tension. It is believed that a source of high amplitude, high frequency flutter in these belt driven tape cartridges arises from the tensioning of the tape as it is wound onto the take-up reel hub. The mechanism by which it is believed tension in the tape wound onto the reel hub creates flutter in the tape velocity as it moves past the recording and/or reproducing means is as follows. As tape is unwound from a reel hub, the tape located more centrally in that same reel hub progressively becomes exposed. As the tape, wound under tension, becomes exposed, it then becomes possible for it to relax and relieve the tension which has been stored in it. Because of the nature of the magnetic tape itself, and the frictional contact existing between successive layers of tape wound onto the tape reel hub, this relaxation process does not always occur smoothly but can occur in bursts as the tape is wound off the reel hub. During such a burst of relaxation, the tape which is undergoing relaxation will become somewhat shorter and somewhat thicker thereby increasing the tension in the off reel tape and momentarily slowing down its motion across the recording and/or reproducing means. Thus, such high frequency flutter is inherent in these cartridges because of the frictional properties of the magnetic tape used in them and the tape tensioning forces created by the flexible belt driving means. Thus, without some improvement either in the frictional properties of the magnetic tape or the structure of the belt driven tape cartridge, it is difficult, if not impossible, to record and/or reproduce digital data at high linear densities for repeated usage.
A further difficulty with currently manufactured tape cartridges is that they tend to damage the edges of the tape as the tape is wound from one tape reel hub to the other tape reel hub. Due to manufacturing tolerances and misalignments in the tape path, the tape approaching tape guides in the tape path does so at an angle from the theoretical perfect alignment. It is common for the misalignment to be in the order of one degree or greater. With tape cartridges heretofore available this has resulted in damage to the tape edge by the guides urging the tape back into alignment. The damage becomes significantly pronounced when the cartridge is loaded with thin magnetic tape, e.g. tape of a 0.5 mil thickness base. The damage to the edges of thin based tape is so severe that complete stoppage of tape movement within the cartridge frequently occurs after approximately 100 to 300 end-to-end transport cycles of tape having a 0.5 mil base. The misalignment is also projected in deflection of the tape from a parallel position to the recording/reproducing bead causing reduced recording output on the edge tracks because of poor head-to-tape contact.
Fixed tape guide members are known in the art of magnetic tape recording and reproducing apparatus, for example, see U. S. Pat. No. 3,270,936, issued to Selsted, et al. However, the tape-guider members of the Selsted patent were especially adapted for use in recorders with removable reels of magnetic recording tape, and the tape guide has conically shaped end pieces in order to facilitate easy threading of magnetic recording tape through the apparatus and for guiding the travel of the magnetic recording tape through the apparatus and for guiding the travel of the magnetic recording tape back onto the right cylindrical body if the magnetic recording tape becomes improperly positioned on the tape guide member. In belt driven tape cartridges, because the magnetic recording tape is contained in a protective enclosure, because the cylindrical guiding flanges placed at the terminal ends of the cylindrical body are essentialy normal to the cylindrical body at the terminal ends, and because of the constant tension in the magnetic recording tape produced driving belt, it is impossible for the magnetic recording tape in a properly manufactured and assembled cartridge to become so improperly positioned on the tape-guide members that the conical surface of the Selsted guide member would ever function as a guide to restore the travel of the magnetic recording tape back onto the right cylindrical body portion of the tape-guide member.